Radiographic and Ultrasonographic Diagnosis of Urolithiasis in Dogs and Cats

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Key Takeaways

• Radiography reliably detects radiopaque stones (calcium oxalate, calcium phosphate, struvite, silica) by x-ray attenuation, appearing as opacities, but misses radiolucent stones like urate and xanthine. Ultrasonography detects all stone compositions via acoustic impedance mismatch and distal acoustic shadowing, making it superior for radiolucent stones.
• The choice of imaging modality is guided by suspected stone composition, with radiography preferred for likely mineralized stones and ultrasonography for suspected urate or xanthine uroliths, based on signalment, urine pH, and crystal morphology.
• Ultrasonography is more sensitive for identifying concurrent abnormalities like bladder wall thickening and ureteral dilation, but its anatomic coverage is limited by acoustic windows, making ureters often poorly visualized unless dilated. Radiography surveys the entire urinary tract but struggles with ureteral stones due to obscuration by surrounding structures.
• Small uroliths (1-2 mm for mineralized stones on radiography, 2-3 mm on ultrasound) can evade detection, and a negative study does not exclude urolithiasis, necessitating further investigation with contrast studies or advanced imaging when clinical suspicion remains high.
• Melamine-associated uroliths are radiolucent and inconsistently visualized on ultrasound, posing a diagnostic challenge; suspected cases require consideration of contrast radiography or advanced imaging if initial studies are unrewarding.
• Documentation of stone number, location, size, shape, margination, and radiographic density or ultrasonographic shadowing is critical for monitoring dissolution therapy and guiding management decisions, with serial imaging protocols needing to be consistent.

This article compares radiography and ultrasonography for the detection of uroliths in dogs and cats, with emphasis on the physical principles that determine stone visibility, the limitations imposed by stone composition and size, and the diagnostic reasoning that guides modality selection. The content serves practicing veterinarians who must choose between imaging techniques, interpret equivocal findings, and understand when a negative study does not exclude urolithiasis. The discussion draws on the ACVIM consensus recommendations on urolith treatment and prevention and standard veterinary imaging resources.

At a Glance

Physical Principles of Stone Detection

Radiographic visibility of a urolith depends on the difference in x-ray attenuation between the stone and its surrounding soft tissue. Stones composed of calcium oxalate, calcium phosphate, struvite, and silica contain elements of higher atomic number than soft tissue and therefore attenuate the x-ray beam more strongly. These stones appear radiopaque. Urate, xanthine, and cystine stones contain only low-atomic-number elements and attenuate the beam similarly to soft tissue, making them radiolucent or only faintly visible on survey radiographs. Melamine-containing stones are also radiolucent, as documented in the context of melamine toxicity and associated stone formation.

Ultrasonography does not depend on atomic number. It detects stones by their acoustic impedance mismatch with urine and soft tissue. A stone surface reflects virtually all incident sound, producing a strongly echogenic interface with distal acoustic shadowing. This mechanism detects all stone compositions, including those that are radiolucent. The practical consequence is that ultrasonography and radiography are complementary instead of interchangeable, and the choice of modality should be guided by the mineral type suspected from signalment, urine pH, and crystal morphology.

Radiographic Technique and Interpretation

Survey radiography of the urinary tract requires two orthogonal views of the abdomen, typically a right lateral and a ventrodorsal projection. A left lateral view may be added when the bladder is positioned within the pelvic canal or when the right lateral view is inconclusive. The entire urinary tract from the kidneys to the urethral orifice must be included on the study. The MSD Veterinary Manual provides species-specific guidance on positioning and normal radiographic anatomy that should be consulted before interpretation.

Mineralized uroliths appear as well-defined, smoothly marginated opacities within the renal pelves, ureters, bladder, or urethra. Multiple stones may be present, and their number, size, shape, and location should be recorded. Stones within the bladder are often mobile and will settle dependently, so their position changes between lateral and dorsoventral views. A stone that does not change position may be lodged in the urethra, embedded in the bladder wall, or located within a diverticulum.

The principal limitation of radiography is the radiolucent stone. Urate stones, which are common in Dalmatians and in cats with portosystemic shunts, are invisible on survey films. Cystine stones are often faintly opaque but may be missed when small. The ACVIM consensus recommendations emphasize that a negative survey radiograph does not exclude urolithiasis and that contrast studies or ultrasonography should be pursued when clinical suspicion remains high.

Ultrasonographic Technique and Interpretation

Ultrasonography of the urinary tract is performed with a microconvex or linear transducer operating at 5 to 10 MHz in dogs and 7.5 to 12 MHz in cats. Higher frequencies improve spatial resolution but reduce penetration, so the transducer choice must balance depth against detail. The patient is positioned in dorsal recumbency, and the bladder is examined through the ventral abdominal wall. The kidneys are evaluated in both sagittal and transverse planes, and the ureters are traced from the renal pelves to the bladder when feasible.

A urolith appears as a hyperechoic focus with distal acoustic shadowing. The shadow is the key diagnostic feature, because it distinguishes a stone from a blood clot, a polyp, or a gas bubble, all of which may be echogenic but do not consistently shadow. Small stones may not produce a visible shadow, particularly when they lie within the bladder lumen surrounded by urine, because the strong specular reflection from the bladder wall can obscure the shadow. In this situation, reducing gain, changing the angle of insonation, or using a higher-frequency transducer can improve shadow detection.

Ultrasonography is superior to radiography for detecting radiolucent stones and for identifying concurrent abnormalities such as bladder wall thickening, mucosal irregularity, and ureteral dilation. It is inferior for surveying the entire urinary tract, because the ureters are often not visible unless dilated, and stones within the pelvic urethra may be obscured by the pelvic floor. The ACVIM consensus recommendations note that ultrasonography is the preferred initial imaging modality for suspected urate or xanthine urolithiasis, whereas radiography is preferred when a mineralized stone is likely.

Modality Selection by Clinical Scenario

The choice of imaging modality should follow a diagnostic algorithm based on signalment, urine pH, and crystal type. For a young Dalmatian with acidic urine and urate crystals, ultrasonography is the primary modality because urate stones are radiolucent. For a middle-aged Miniature Schnauzer with alkaline urine and struvite crystals, survey radiography is sufficient because struvite stones are radiopaque. When the mineral type is unknown, radiography should be performed first because it surveys the entire tract, and ultrasonography should follow if the radiograph is negative or if radiolucent stones are suspected.

Ureteral stones present a particular challenge. Mineralized ureteral stones may be visible radiographically, but they are often obscured by fecal material, osseous structures, or the small size of the stone. Ultrasonography can identify a dilated ureter proximal to a stone, but the stone itself may be difficult to image when the ureter is not dilated. In both modalities, a negative study does not exclude a ureteral stone, and advanced imaging such as computed tomography should be considered when clinical signs are severe or progressive.

Imaging Findings by Stone Type

Calcium Oxalate and Calcium Phosphate

Calcium-containing uroliths are the most reliably detected stones with both modalities. On radiographs, calcium oxalate monohydrate and dihydrate stones appear sharply marginated, round to irregular, and intensely opaque. Their mineral density approaches or exceeds that of bone, so even small stones of 1 to 2 mm diameter are usually visible when projected free of superimposition. Calcium phosphate stones, including apatite and brushite, show similar radiopacity, though brushite tends to be less uniformly dense.

On ultrasound, calcium oxalate stones produce a strong hyperechoic interface with distal acoustic shadowing. Shadowing is typically complete and dense, which helps distinguish them from soft tissue masses or blood clots. A pitfall arises with very small stones, under approximately 3 mm, where the acoustic shadow may be subtle or absent. In this setting, a hyperechoic focus without shadowing should still be considered suspicious, and radiography should be performed for confirmation.

Struvite

Struvite stones are moderately radiopaque. Their density is lower than calcium oxalate but usually sufficient for detection on survey radiographs when the stone exceeds 3 to 4 mm. Larger struvite stones, particularly those filling the bladder lumen or forming a staghorn configuration in the renal pelvis, are readily identified. The radiographic appearance is often smooth and ovoid, and multiple stones are common.

Ultrasonographically, struvite stones are hyperechoic with variable shadowing. The shadow may be less complete than that seen with calcium oxalate, particularly in smaller stones. Struvite sand or fine gravel can appear as a layer of echogenic debris along the dependent portion of the bladder, sometimes without discrete shadowing. This finding can be mistaken for cystitis or sediment, and a radiograph is useful to confirm mineral content.

Urate and Cystine

Urate stones, including ammonium urate and sodium urate, are radiolucent on standard radiographs. They are not visible unless they have acquired a mineral coating or have become large enough to displace adjacent structures. This limitation is clinically significant because urate uroliths are common in Dalmatians, English Bulldogs, and cats with portosystemic shunts. Ultrasound is the primary imaging modality for these stones, where they appear as hyperechoic foci with distal shadowing, often with a smooth contour.

Cystine stones are variably radiopaque. Pure cystine stones are typically faintly opaque or radiolucent, while those with calcium coprecipitation may be visible. The radiographic density of cystine is lower than struvite and considerably lower than calcium oxalate. Ultrasound detection is reliable for stones of sufficient size, but small cystine stones may be missed. In breeds predisposed to cystinuria, such as Newfoundland dogs, a negative radiograph does not exclude urolithiasis.

Silica and Mixed Composition

Silica stones are moderately radiopaque, similar to struvite, and are usually detectable radiographically when larger than 3 mm. They often have a jackstone appearance with spiculated margins, though this is not pathognomonic. Mixed-composition stones display radiographic density determined by their most attenuating component. A stone with a calcium oxalate shell over a urate core will appear radiopaque, while a predominantly urate stone with minimal calcium will remain radiolucent.

Melamine and Toxin-Related Stones

Melamine-associated uroliths, recognized after the 2004 and 2007 pet food contamination events, are radiolucent and inconsistently visualized on ultrasound. The stones form when melamine co-ingests with cyanuric acid, and they may be composed of melamine-uric acid complexes. As described in the toxicology literature, these stones are not consistently seen on ultrasound, which creates a diagnostic gap for affected animals. Melamine toxicity emphasizes that clinical signs are nonspecific and imaging findings may be negative despite significant stone burden. In suspected toxin exposure, contrast radiography or advanced imaging should be considered when survey radiographs and ultrasound are unrewarding.

Anatomic Site Considerations

Renal and Ureteroliths

Renal pelvic stones are best assessed with ultrasound, which can identify pelvic dilation, peripelvic fibrosis, and concurrent hydronephrosis. Radiography detects radiopaque nephroliths but cannot assess obstruction severity. Ureteroliths present a greater challenge. Small ureteral stones are frequently obscured by fecal material, osseous structures, or patient positioning on radiographs. Ultrasound can identify ureteral stones when the ureter is dilated, but a non-dilated ureter with a small stone may escape detection. The ACVIM consensus recommendations acknowledge that imaging of ureteral calculi is technically demanding and that a combination of modalities is often required for definitive diagnosis. ACVIM consensus recommendations on urolith treatment and prevention

Cystic and Urethral Stones

Bladder stones are the most straightforward to image. A full bladder on radiography displaces surrounding viscera and provides contrast against the bladder wall. On ultrasound, a moderately distended bladder allows complete evaluation of the lumen and wall. Urethral stones in male dogs are often palpable or visible radiographically within the penile urethra, but those in the membranous or prostatic urethra require careful imaging. Retrograde urethrography or positive-contrast cystography may be necessary when survey radiographs are negative and clinical signs strongly suggest urethral obstruction.

Decision Table for Imaging Modality

Documentation and Reporting

Imaging reports should describe stone number, location, size, shape, margination, and radiographic density relative to bone. For ultrasound, report the acoustic shadowing characteriztics, the presence of pelvic or ureteral dilation, and any concurrent bladder wall thickening. Measurements should be recorded in at least two orthogonal planes for radiography and in the plane of maximum diameter for ultrasound. When stones are detected incidentally, the report should note that mineral density does not predict composition and that stone analysis is required for definitive identification.

Serial imaging is indicated for monitoring dissolution therapy. The ACVIM consensus recommendations support repeat imaging at defined intervals to assess stone reduction, though the optimal interval depends on stone type and treatment protocol. ACVIM consensus recommendations on urolith treatment and prevention notes that imaging findings guide decisions to continue, modify, or terminate medical therapy. Radiography is preferred for monitoring radiopaque stones because it is less operator-dependent than ultrasound and provides consistent measurements. Ultrasound is used when stones are radiolucent or when concurrent assessment of the renal pelvis and ureters is needed.

Species and Equipment Considerations

Feline patients present specific challenges. Their smaller body size improves radiographic detail, but the frequency of radiolucent urate and cystine stones in cats means that a negative radiograph carries less weight than in dogs. Feline ureteroliths are often small and located within the distal ureter, where they are difficult to image with either modality. Ultrasonography in cats requires higher-frequency transducers, typically 10 to 18 MHz, to achieve adequate resolution, and the smaller bladder volume limits the acoustic window.

In dogs, body conformation affects image quality. Deep-chested breeds may have renal and ureteral structures positioned dorsally, requiring oblique projections for radiography. Obese patients degrade both radiographic contrast and ultrasound penetration. Portable radiography units with lower output may not penetrate large canine abdomens adequately, and referral to a higher-output fixed unit should be considered when image quality is marginal.

The availability of digital radiography has improved contrast resolution compared with film-screen systems, allowing detection of fainter stones such as cystine. However, digital systems can also introduce artifacts, including edge enhancement that mimics or obscures small stones. Ultrasound equipment with tissue harmonic imaging improves stone detection in the bladder, particularly for small or weakly shadowing stones. When only a single modality is available, the choice should default to ultrasound for suspected radiolucent stones and radiography for suspected calcium-containing stones, with the understanding that a negative study does not exclude urolithiasis.

Recognized Complications and Early Detection

Imaging-guided management of urolithiasis carries several recognized failure modes. The most consequential is the missed stone. A radiopaque calculus hidden within the renal silhouette on a poorly penetrated radiograph, or a small urethral stone obscured by the os penis in male dogs, can escape detection and progress to obstruction. Ultrasonography reduces this risk for the bladder and proximal urethra but introduces its own blind spots, particularly the penile urethra and the intramural ureteral segment.

Pneumocystography or contrast cystography should be performed when survey radiographs are negative but clinical signs strongly suggest cystic calculi. The ACVIM consensus recommendations on urolith treatment and prevention emphasize that imaging findings must be paired with sediment analysis and culture, because radiographic appearance alone cannot reliably predict stone composition and therefore cannot guide dissolution therapy.

Early detection of obstruction relies on serial imaging. Ultrasonographic evidence of renal pelvic dilation, ureteral dilation, or increased resistive index in the affected kidney should prompt immediate decompression. In cats with suspected ureteroliths, a normal renal pelvis on a single examination does not exclude intermittent obstruction. Repeat ultrasonography within 24 to 48 hours, or after fluid diuresis, is warranted when clinical suspicion remains high.

Common Errors and Corrective Actions

Less experienced clinicians frequently misclassify radiolucent stones as absent. Urate, cystine, and xanthine calculi are poorly radiopaque or entirely radiolucent on survey radiographs. The corrective step is to perform contrast studies or ultrasonography whenever hematuria, dysuria, or recurrent urinary tract infection persists despite a negative survey study. Conversely, phleboliths, intestinal contents, and osseous metaplasia of the bladder wall can be mistaken for cystic calculi. The discriminating feature is mobility: true cystic calculi change position with patient repositioning, whereas mural or extravesical mineralisation does not.

Another common error is over-reliance on stone size as a predictor of passage. A 3 mm urethral calculus in a male cat may cause complete obstruction, while a similar stone in a female dog may pass spontaneously. The ACVIM consensus recommendations note that stone size must be interpreted in the context of patient sex, urethral diameter, and clinical signs, not as an absolute threshold.

A third error is failure to obtain orthogonal views. A stone superimposed on the vertebral column or pelvic canal on the ventrodorsal view may be invisible, and the lateral view alone can miss stones in the urethral bulb. Two views are the minimum standard, and oblique or stressed views are indicated when the clinical picture does not match the imaging findings.

Limitations of Current Evidence

The comparative accuracy of radiography versus ultrasonography for urolith detection in dogs and cats rests on a modest evidence base. Most published studies are retrospective, use different reference standards, and report sensitivity figures that vary with stone size, location, and operator experience. The ACVIM consensus recommendations acknowledge that no single imaging modality detects all uroliths and that the choice of modality should be guided by stone location, suspected composition, and patient factors.

Expert opinion still differs on the role of advanced imaging. Some specialists advocate computed tomography for all recurrent stone formers, citing its superior sensitivity for small ureteroliths and radiolucent stones. Others argue that ultrasonography combined with contrast radiography achieves comparable diagnostic yield at lower cost and without general anesthesia in most patients. The evidence does not yet resolve this debate, and the clinician must weigh local availability, patient stability, and owner resources.

Melamine-associated stones illustrate a further limitation. These stones may be radiolucent and inconsistently visualized on ultrasound, as documented in the melamine toxicity literature. When toxin exposure is suspected, imaging must be supplemented by dietary history and, where indicated, toxicologic analysis of the stone or feed.

Referral and Escalation Criteria

Referral to a veterinary radiologist or internal medicine specialist is appropriate when ultrasonographic findings are equivocal, when a ureterolith is suspected but not confirmed, or when recurrent stone formation occurs despite appropriate medical management. Interventional procedures such as lithotripsy, ureteral stenting, or cystoscopic stone retrieval require specialised equipment and training, and the ACVIM consensus recommendations describe these as options best performed by clinicians experienced in their use.

Laboratory involvement is essential when stone composition cannot be inferred from imaging and medical dissolution is contemplated. Quantitative stone analysis should be performed on any retrieved calculus, and urine culture with susceptibility testing is indicated before and after stone removal. The MSD Veterinary Manual provides species-specific guidance on sample handling and interpretation of urine sediment findings.

Regulatory reporting is rarely required for urolithiasis in companion animals. An exception arises when a cluster of stone cases is linked to a commercial diet or a suspected contaminant. The AVMA practice resources and WOAH terrestrial animal health standards describe reporting pathways for suspected feed-related toxicoses and notifiable diseases respectively. Practitioners should contact their regional veterinary diagnostic laboratory for guidance when such a cluster is suspected.

Troubleshooting Table

Frequently Asked Questions

How should I proceed when only one imaging modality is available?

When only radiography is available, obtain orthogonal views and use a high-detail screen-film system or digital detector. Radiolucent stones such as urate, cystine, and melamine-associated concretions will be missed, so pursue contrast cystography or urethrography when clinical signs persist despite negative survey films. When only ultrasound is available, scan the bladder in transverse and sagittal planes with the patient in dorsal and lateral recumbency. Small stones near the trigone can hide behind acoustic shadowing from the pelvic brim. Move the bladder by gentle compression or repositioning. The ACVIM consensus recommendations on urolith treatment and prevention note that imaging findings should guide, not replace, stone analysis whenever retrieval is feasible.

Which imaging findings change my medical management decisions?

Stone size, number, and location determine whether dissolution, dietary modification, or intervention is appropriate. A single small cystic stone in a female dog may pass spontaneously, while multiple large stones or urethral obstruction require removal. Radiographic opacity helps predict composition, which informs dissolution candidacy. Struvite and urate stones may dissolve medically, whereas calcium oxalate and silica stones do not. Ultrasound identifies stones too small for radiography, which matters when recurrence prevention is the goal. The ACVIM consensus recommendations emphasize that imaging alone cannot confirm composition, and submitting retrieved stones for quantitative analysis remains the standard for targeted prevention.

What should I document in the medical record for serial monitoring?

Record the imaging modality, patient positioning, and the number, size, location, and radiographic opacity or ultrasonographic appearance of each stone. Measure the largest stone dimension in two planes. Note the presence of shadowing, bladder wall thickness, and any concurrent urinary tract changes. For serial comparisons, replicate the same modality, transducer frequency, and patient positioning. Store representative images instead of only written descriptions. Document whether the bladder was distended or partially filled, since this affects stone visibility and measurement. The ACVIM consensus recommendations support using consistent imaging protocols to track dissolution or growth over time.

How do I explain imaging limitations to a client whose pet has a normal radiograph but persistent signs?

Explain that some stone types are invisible on standard radiographs because their mineral density is similar to soft tissue. Urate and cystine stones fall into this category. Ultrasound can detect many of these, but very small stones may still escape detection. A negative imaging study does not exclude urolithiasis, and persistent hematuria, dysuria, or stranguria warrants further investigation. Contrast studies or advanced imaging may be indicated. The MSD Veterinary Manual describes radiolucent uroliths as a recognized diagnostic challenge in small animal practice. Frame the discussion around the clinical picture, not the imaging result alone, and outline the next diagnostic step clearly.

Does imaging differ for cats compared with dogs?

Cats present specific challenges. Their smaller bladder capacity makes adequate distension harder to achieve, which reduces ultrasound sensitivity for small stones. Feline urethral stones are often radiopaque but may be obscured by the os penis or pelvic bones on survey radiographs. Ultrasonography of the feline urethra is limited by its length and narrow diameter, so positive-contrast urethrography remains valuable when urethral obstruction is suspected. Renal mineralisation is more commonly detected incidentally in cats, and ultrasound is superior to radiography for identifying small nephroliths. The ACVIM consensus recommendations address species-specific differences in stone composition and imaging approach.

When should I refer for advanced imaging or specialist consultation?

Refer when imaging findings are inconclusive but clinical signs persist, when recurrent urolithiasis suggests an underlying metabolic disorder, or when stone burden is complex and surgical planning requires precise localization. Refer also when ureteral stones are suspected, since their detection and management are technically demanding. The American College of Veterinary Radiology provides resources for locating board-certified radiologists and understanding specialty imaging standards. Early referral is preferable to repeated inconclusive studies, particularly when the patient is male, obstructed, or has declining renal function. Provide the consultant with prior images and a summary of clinical findings to avoid redundant testing.

Related Clinical & Scientific Guides

• MRI Monitoring of Brain Tumor Response to Therapy in Dogs
• Ultrasound-Guided Drainage of Abscesses in Small Animals
• Radiographic Monitoring of Total Hip Replacement in Dogs

References and Further Reading

• ACVIM Small Animal Consensus Recommendations on the Treatment and Prevention of Uroliths in Dogs and Cats.. 2016.
• Melamine toxicity.. 2010.
• Pelvic floor muscle training versus no treatment, or inactive control treatments, for urinary incontinence in women.. 2018.
• A guide for urogynecologic patient care utilizing telemedicine during the COVID-19 pandemic: review of existing evidence.. 2020.
• A Drosophila model identifies a critical role for zinc in mineralization for kidney stone disease.. 2015.
• American College of Veterinary Radiology Resources. American College of Veterinary Radiology.
• MSD Veterinary Manual, Professional Edition. MSD Veterinary Manual.
• American Veterinary Medical Association Practice Resources. American Veterinary Medical Association.
• WOAH Terrestrial Animal Health Code. WOAH.

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This article is educational professional reference material for veterinary audiences. It is not a substitute for veterinary diagnosis, individual clinical judgment, current product labeling, or applicable regulatory requirements.